PURPOSE: To permit the flow rate of a fluid to be accurately measured in the whole area by alternately delivering a fluid to both cylinder chambers of a cylinder containing a free piston, and counting the number of the reciprocation of the piston.

CONSTITUTION: When a solenoid valve 17 is not excited (at a position A), the fuel delivered from a fuel pump 18 flows into a cylinder 10 from the left. This causes a piston 11 to move rightwards, and consequently the fuel having filled a chamber 10B is supplied through a port 16 to a carbureter float chamber 19. The movement of the piston 11 at that time is sensed by a sensor 13' comprising a light-emitting element 13a' and a light-receiving element 13b'. By the sensing signal, the solenoid valve 17 is excited as well as a counter 24 is actuated. At this time, the fuel flows through the port 16, causing the piston 11 to move leftwards. Consequently, the fuel having filled a chamber 10A is supplied to the carbureter float chamber 19. Counting the signals from the sensors 13 and 13' by using the counter 24 permits the flow rate of the fuel to be accurately measured.